1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls that include a center, an envelope layer, a mid layer, and a cover.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver, a long iron, and a middle iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.
Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.
In light of achieving various performance characteristics, golf balls each having a multilayer structure have been proposed. JP10-328326 (U.S. Pat. No. 6,468,169) discloses a golf ball that includes a core, an envelope layer, an inner cover, and an outer cover. JP2001-17575 (U.S. Pat. No. 6,271,296) discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover. JP2002-272880 (U.S. Pat. No. 6,913,547) discloses a golf ball that includes a core and a cover. The core consists of a center and an outer core layer. The cover consists of an inner cover layer and an outer cover layer. JP2003-205052 (US 2003/0166422) discloses a golf ball that includes a center, a mid layer, and a cover. JP2004-130072 (US2004/0029648) discloses a golf ball that includes a core and a cover. The core has a three-layer structure.
When a core that has an outer-hard/inner-soft structure and an excessive hardness distribution is hit with a driver, the energy loss is high in the core. The energy loss impairs resilience performance. When a core that has an outer-hard/inner-soft structure and an excessive hardness distribution is hit with a short iron, the spin rate is low. The low spin rate impairs controllability.
An object of the present invention is to provide a golf ball that provides a large flight distance when being hit with a driver and that has excellent controllability when being hit with a short iron.